San Pablo Dam Seismic Upgrades

Alternative solutions were evaluated based on the seismic study and review by EBMUD and the state Division of Safety of Dams (DSOD). Deep Soil Mixing (DSM) was chosen to mitigate liquefaction in the downstream toe of the dam foundation. This application of the DSM process involves strengthening the soil by mixing cement grout with existing soil to form grids of walls in the ground. These grids limit shear strains in the soil during earthquakes to mitigate liquefaction.

DSM allowed the reservoir to remain used during construction with minimal environmental impact. The dam's existing stability berm and downstream toe were graded to provide work pads for DSM. Since holes are not drilled, soil stockpiles are minimized on the small site. DSM also minimized the need to borrow fill for the added stability berm placed on top of the DSM zones. Fill came from the adjacent hills, avoiding off-site import.

Project Highlights

• Improve the embankment and foundation soils downstream of San Pablo Dam to withstand the shaking generated by the maximum credible earthquake on the Hayward Fault Zone, prevent the release of reservoir water, and protect public safety.

• Triple-axle DSM equipment improved soils to an average depth of 80 ft. Two DSM rigs and pre-drilling rigs, working double shifts, were used on the project site.

• DSM construction exceeded the unconfined compressive strength and soil mix uniformity design requirements. The spacing of DSM walls varied the design specifications for the unconfined compressive strength of shear walls.

• This project won the ASCE Region 9 Outstanding Geotechnical Project of the Year 2009 Award.